An extensible type to record the type of trees used as backend for the lazy map.

Note: If you use the 'tree-encoding' library, then its functor takes care of adding a new constructor for the expected tree.

KeyS is the qualifier signature for key types in the lazy map. Externally visible and accessible keys of the lazy map are always non-negative. However, the lazy map implementation may internally use negative keys therefore modules of type KeyS must support them.

UnexpectedAccess is raised in the default of the produce_value argument to S.create.

Make generates a lazy map module using a given Key module.